Polarimetry in Astronomy

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Polarimetry in Astronomy Or Do you know where your photons are coming from? Elizabeth Corbett AAO

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Polarimetry in Astronomy. Or Do you know where your photons are coming from ?. Elizabeth Corbett AAO. Polarimetry: The Basics. Light be described in terms of two components:. Taken from Hecht (1987) “Optics”. Polarimetry: The Basics. In general light is elliptically polarised. - PowerPoint PPT Presentation

Transcript of Polarimetry in Astronomy

Page 1: Polarimetry in Astronomy

Polarimetry in Astronomy

Or

Do you know where your photons are coming from?

Elizabeth CorbettAAO

Page 2: Polarimetry in Astronomy

Polarimetry in Astronomy 2

Polarimetry: The Basics

)cos( tkExx zE

)cos( tkE yy zE

yx EEE

Light be described in terms of two components:

Taken from Hecht (1987) “Optics”

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Polarimetry: The Basics

Special cases:

= 0 or nlinearly polarised light

= /2 n and Ex=Ey circularly polarised light

Unpolarised light has a well-defined E which fluctuates rapidly, hence no net polarisation is measured.

In general light is elliptically polarised

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Introducing: - The Stokes Vectors

• Electromagnetic radiation can be described in terms of the Stokes Vectors; I, Q, U & V.

– I - total intensity– Q & U - describing linear polarisation– V - circular polarisation

• Polarisation PA:

• Degree of Polarisation:

– For linear polarisation V=0

Q

U1tan2

1

I

VUQ 2 22 P

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Why Stokes Vectors?

• Easy to describe polarisations:

• Additive - e.g.

light polarisedcircular right 100% 1,0,0,1

)(90at polarisedlinearly 100% )0,0,1,1(

45at polarisedlinearly 100% 0,1,0,1

)(0at polarisedlinearly 100% (1,1,0,0)

light ed)(unpolaris Natural 0,0,0,1

o

o

o

S

S

S

S

S

WestEast

SouthNorth

)0,0,0,2()0,0,1,1()0,0,1,1(

%705.22 )0,1,1,2()0,1,0,1()0,0,1,1(

)0,0,1,1()0,0,0,9()0,0,1,10(

21

021

SS

SS

S

P

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Sources of Polarised Emission • Synchrotron:

– dominant radiation mechanism in the optical - radio continua of the blazar class of AGN, also seen in SNR, pulsars

– emitted by charged particles, generally electrons accelerated by a magnetic field

– produces a high degree of linear polarisation (up to 45% in some blazars)

– polarisation position angle is aligned with the E vector perpendicular to the local magnetic field

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• Dichroic Absorption:

– also known as interstellar polarisation

– dichroic absorbers preferentially absorb radiation with one polarisation state and transmit the orthogonal state

– due to anisotropic dust grains aligning in the presence of a magnetic field

– radiation passing through such a cloud becomes polarised with an E vector parallel to the magnetic field

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• Scattering:– Light can be scattered by electrons or dust– High degrees of linear polarisation can result– Polarisation PA is perpendicular to the scattering plane– Degree of polarisation depends on the scattering angle,

– Circular polarisation can result from multiple scatters from dust

100%

0%

60%

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Polarisation in AGN

Absorbed by torus

Synchrotron emission

Direct view

Dichroic absorptionby dust

BLR

NLR

Torus

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Synchrotron

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Polarisation in AGN

Scattered off torus

Absorbed by torus

Synchrotron emission

Scattered in the NLR

Direct view

Dichroic absorptionby dust

BLR

NLR

Torus

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Scattered in the NLR

Circinus: Alexander et al (2000)

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Young et al (1996)

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Polarisation in AGN

Scattered off torus

Absorbed by torus

Synchrotron emission

Scattered in the NLR

Direct view

Dichroic absorptionby dust

BLR

NLR

Scattered by moving scatterers

Torus

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Scattered by moving scatterers

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Polarimeter

To spectrograph or imager

To TV guider

Tilted slit/dekker

Arc lamp

/2 plate

Analyser

Calcite

/4 plate

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Polarimetry

} Object {

} Sky {oe

oe

oeoe

SpectroscopyImaging

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• Rotate the wave-plate to 0o, 45o, 22.5o and 67.5o

– Subtract the sky from each ray on each frame

– Take the ratio of the intensities of the o- and e- ray for each frame

– For Q, and

– For U, and

– Polarization PA

– Degree of polarization

Measuring the Stokes Parameters

45

02

I

IRQ

1

1

U

U

R

RU

5.67

5.222

I

IRU

1

1

Q

Q

R

RQ

Q

U1tan2

1

I

UQ 2 2P

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Summary

• Polarimetry provides information on where your photons originated– Have they been scattered?

– Have they been through dust?

– Have they (perhaps) come from a jet?

• Important for inclination dependent systems - eg AGN, YSO

• “Not as hard as it used to be” - easy data reduction

• But - very “photon hungry”

– so for a P~0.1% you need SNR ~1400 or 2E6 photons!

SNRP

2100

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Scattered by NLR Scattered by torus